The present invention relates to a trimming circuit for use in, for example, an electronic circuit such as a semiconductor device and for adjusting the electrical characteristics of the electronic circuit.
Japanese Patent Laid-Open Publication No. 2004-79158 describes a trimming circuit that is used to finely adjust the characteristics of an electronic circuit such as a semiconductor device. In this patent publication, a trimming circuit is used as a deviation temperature detection circuit of a temperature detector. The trimming circuit described in the patent publication may have the configuration shown in FIG. 5. The trimming circuit shown in FIG. 5 has series-connected units ua1 to ua6. The units ua1 to ua6 respectively have 32, 16, 8, 4, 2, and 1 series-connected resistors R. The resistors are connected in parallel to transistors that perform switching operations. This configuration makes it possible to control the transistor of each of the units to change the resistance in a stepped manner in a range of 0 to 64 times the unit resistance (the resistance of each resistor R) to perform trimming. This trimming circuit, however, requires as many as 63 resistors R.
As shown in FIG. 6, a trimming circuit may be configured by connecting resistors R in parallel. In this trimming circuit, units ub1 to ub6 are connected to each other in series. In the unit ub1, two series-connected resistors R are connected to a transistor in parallel. In the unit ua2, a resistor R is connected to a transistor in parallel. In the units ub3 to ub6, two, four, eight and sixteen series-connected resistors R are respectively connected in parallel to a transistor, which performs a switching operation. This configuration makes it possible to control the transistor of each of the units to change the resistance in a stepped manner in a range of 1/16  to 2 times the unit resistance (the resistance of each resistor R). The trimming circuit is formed by a smaller number of resistors R than the trimming circuit in FIG. 5, yet requires as many as 33 resistors R.
As described above, a trimming circuit requires many resistors R. Moreover, in the trimming circuit shown in FIG. 6, the resistors R are connected in parallel to transistors, which perform switching operations, in the units ub2 to ub6. Therefore, when the resistance that is adjusted is low in a unit, the resistance Ron for when the transistor is on will affect the resistance. For example, in the unit ub6 of which adjusted resistance is minimal, the transistor is connected in parallel to 16 resistors R. Assuming that the resistance Ron of the transistor is R/16, the resistance of the unit ub6 when the transistor is turned on will be R/32. Although the target resistance of the unit ub6 is set to zero when the transistor is off and set to R/16 when the transistor is on, the actual resistance is only R/32 when the transistor is off and R/16 when the transistor is on. Thus, in the prior art, the actual adjusted resistance differs greatly from the desired resistance due to the activation and inactivation of the transistor.